Forward osmosis membrane for seawater desalination and method for preparing the same

ABSTRACT

A forward osmosis membrane for seawater desalination and a method for preparing the same. The forward osmosis membrane has a composite membrane structure including a nonwoven fabric layer, a hydrophilic polymer layer, and a polyamide layer. The hydrophilic polymer layer formed on the nonwoven fabric layer facilitates an inflow of water from the feed water to the draw solution to enhance flux and realize high water permeability in the direction of osmosis. The polyamide layer not only secures contamination resistance and chemical resistance but also minimizes the back diffusion of salts of the draw solution in the direction of reverse osmosis. Hence, the forward osmosis membrane of the present invention is greatly useful for desalination of high-concentration seawater.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a National Stage application of InternationalApplication No. PCT/KR2011/000856, filed on Feb. 9, 2011, which claimspriority of Korean Application Serial Numbers 10-2010-0040485 filed onApr. 30, 2010, 10-2010-0067960 filed on Jul. 14, 2010, 10-2010-0109646filed on Nov. 5, 2010 and 10-2010-0129340 filed on Dec. 16, 2010, all ofwhich are incorporated herein by reference in their entireties.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a forward osmosis membrane for seawaterdesalination and a method for preparing the same and, more particularly,to a forward osmosis membrane for seawater desalination and a method forpreparing the same that facilitates an inflow of water from a feed waterto a draw solution to realize high water permeability and excellentcontamination resistance and particularly satisfies the property ofpreventing the back diffusion of solutes of the draw solution in thedirection of reverse osmosis, thereby being suitable for desalinatinghigh-concentration seawater.

2. Description of the Prior Art

Forward osmosis is a membrane separation technique that uses the osmoticpressure caused by the difference in concentration between two solutionsas a driving force for transport of water from lower-concentrationsolution to higher-concentration solution through a membrane. As theforward osmosis is just the opposite of the reverse osmosis, thepreparation of a forward osmosis membrane is also distinctive from thatof a reverse osmosis membrane.

The forward osmosis membrane not only facilitates an inflow of waterfrom feed water to a draw solution through the membrane but also playsan important role in maintaining a constant concentration of the drawsolute and a high osmotic pressure. For this, it is of the essence thatthe forward osmosis membrane is designed to have high water permeabilityin the direction of osmosis and not to allow the solutes of the drawsolution diffused in the direction of reverse osmosis. Likewise, themembrane with minimized contamination is given precedence in thepreparation of a forward osmosis membrane. The necessary characteristicsof a forward osmosis membrane are as follows.

Firstly, the support layer provided in the forward osmosis membrane isto have high porosity and low pore tortuosity in order to minimize theinternal concentration polarization and to increase contaminationresistance.

Secondly, the thickness of the forward osmosis membrane is be minimizedin order to increase the flux of water passing through the membrane.

Thirdly, hydrophilic materials are used to minimize water-aidedfiltration resistance.

Fourthly, the solutes of the draw solution are not allowed to diffusefrom higher-concentration solution to lower-concentration solution inorder to maintain the draw solution at high concentration.

As for the conventional methods for preparing a forward osmosismembrane, U.S. Patent No. 2006-0226067 discloses a preparation methodfor a forward osmosis membrane using cellulose triacetate as ahydrophilic material. More specifically, solutions of a same materialand different concentrations were applied on a support layer 25 to 75 μmthick to form a selective layer 8 to 18 μm thick. An evaluation in theforward osmosis (FO) mode using a draw solution, the membrane completedturned out to be a forward osmosis membrane having a high flux of 11GFD. However, the membrane undesirably allowed the solutes of the drawsolution to diffuse from higher-concentration draw solution tolower-concentration feed water. Such a membrane is impractical in thesituation that requires the draw solution to be maintained higher inconcentration than high-concentration feed water such as seawater thathas a great content of salts.

According to the International Patent No. 2008-137082, a polysulfonesolution was cast on a nonwoven fabric to form a membrane like anultrafiltration membrane. On the surface of the membrane thus obtained,a polyamide reverse osmosis membrane was prepared by carrying out aninterfacial polymerization reaction of polyfunctional amine andpolyfunctional acyl halide. The membrane removed of the nonwoven fabricwas applied to the forward osmosis (FO) system. A property assessment inthe FO mode showed that the forward osmosis membrane had a flux of 0.5GFD and a salt rejection rate more than 99%. The forward osmosismembrane secures a salt rejection rate good enough to separatehigh-concentration feed water such as seawater but has limitation inpractical usage because of low flux.

The membrane prepared from a polysulfone-based polymer according to theprior art is excellent in mechanical strength and thermal and chemicalstability and thus can be used as a membrane material. However, thiskind of membrane tends to adsorb contaminants according to thecharacteristic of a hydrophobic membrane, which results in a loss ofseparation function and consequently reduced life span.

SUMMARY OF THE PRESENT INVENTION

It is an object of the present invention to provide a forward osmosismembrane suitable for desalination of high-concentration seawater.

It is another object of the present invention to provide a method forpreparing a forward osmosis membrane for seawater desalination.

To accomplish the above objects, in accordance with a first preferredembodiment of the present invention, there is provided a forward osmosismembrane for seawater desalination that includes a hydrophilic polymerlayer and a polyamide layer.

In accordance with a second preferred embodiment of the presentinvention, there is provided a forward osmosis membrane for seawaterdesalination that has a composite membrane structure of sequentiallylaminated layers including a nonwoven fabric layer, a hydrophilicpolymer layer and a polyamide layer.

The forward osmosis membrane for seawater desalination has aconductivity per minute not more than 9.0 μS/cm per minute (over amembrane surface area of 24 cm²) and shows a low back diffusion ofsalts. Simultaneously, the forward osmosis membrane has a flux of 3 to20 GFD in the presence of a 2M NaCl draw solution or under an equivalentosmotic pressure condition.

In the forward osmosis membrane for seawater desalination of the presentinvention, the nonwoven fabric layer preferably has an air permeabilityat least 2 cc/cm²·sec, an average pore size of 1 to 600 μm, and acontact angle of 0.1 to 74 degrees. Preferably, the nonwoven fabriclayer has a thickness of 20 to 150 μm.

In accordance with the first and second embodiments of the presentinvention, the hydrophilic polymer layer includes any one selected fromthe group consisting of polyacrylonitrile, polyacrylate,polymethylmethacrylate, polyethylene imide, cellulose acetate, cellulosetriacetate, polyvinyl alcohol, polyvinylpyrrolidone, polyethyleneglycol,polysulfone-based polymer, polyethylene oxide and polyvinyl acetate ormixture thereof.

The hydrophilic polymer as a mixture contains any one selected frompolyvinylpyrrolidone, polyvinyl alcohol, polyethyleneglycol or celluloseacetate in an amount of 0.1 to 5 wt % in combination withpolyacrylonitrile.

More preferably, the hydrophilic polymer as a mixture contains 0.1 to 10wt % of a sulfonated polysulfone-based polymer represented by formula 1in combination with the polysulfone-based polymer:

where A is any one functional group selected from:

B is any one functional group selected from:

m/(n+m) is 0.2 to 0.7; and x is 50 to 2,300.

The polysulfone-based polymer is any one selected from the groupconsisting of polysulfone, polyethersulfone and polyarylethersulfone ormixture thereof. More preferably, the sulfonated polysulfone-basedpolymer is a compound represented by formula 2:

where m/(n+m) is 0.2 to 0.7; and x is 50 to 2,300.

In the forward osmosis membrane of the present invention, thehydrophilic polymer layer is characterized by finger-like pores. Thethickness of the hydrophilic polymer layer is preferably in the range of30 to 250 μm.

In the forward osmosis membrane of the present invention, the polyamidelayer is formed by an interfacial polymerization of an aqueous solutioncontaining polyfunctional amine or alkylated aliphatic amine and anorganic solution containing a polyfunctional acyl halide compound.

More preferably, the polyamide layer is formed by an interfacialpolymerization of an aqueous solution being prepared by further adding0.01 to 2 wt % of the polyamine salt compound to an aqueous solutioncontaining polyfunctional amine or alkylated aliphatic amine and anorganic solution containing a polyfunctional acyl halide compound.

The polyamine salt compound is prepared from a tertiary polyamine and astrong acid at a molar ratio of 0.5˜2:1. The tertiary polyamine is anyone selected from the group consisting of 1,4-diazabicyclo[2,2,2]octane(DABCO), 1,8-diazabicyclo[5,4,0]undec-7-ene (DBU),1,5-diazabicyclo[4,3,0]none-5-ene (DBN), 1,4-dimethylpiperazine,4-[2-(dimethylamino)ethyl]morpholine,N,N,N′,N′-tetramethylethylenediamine,N,N,N′,N′-tetramethyl-1,3-butanediamine,N,N,N′,N′-tetramethyl-1,4-butanediamine (TMBD),N,N,N′,N′-tetramethyl-1,3-propanediamine,N,N,N′,N′-tetramethyl-1,6-hexanediamine (TMHD),1,1,3,3-tetramethylguanidine (TMGU) andN,N,N′,N′,N″-pentamethyldiethylenetriamine.

The present invention provides a method for preparing a forward osmosismembrane for seawater desalination having a three-layered structure ofnonwoven fabric layer, hydrophilic polymer layer and polyamide layerthat includes: (a) forming a hydrophilic polymer layer by doping asolution containing 10 to 25 wt % of a hydrophilic polymer on a nonwovenfabric layer; and (b) forming a polyamide layer by an interfacialpolymerization reaction of an organic solution containing apolyfunctional acyl halide compound and an aqueous solution containingpolyfunctional amine or alkylated aliphatic amine on the hydrophilicpolymer layer.

The present invention also provides a method for preparing a forwardosmosis membrane for seawater desalination having a two-layeredstructure of hydrophilic polymer layer and polyamide layer thatincludes: (a) forming a hydrophilic polymer layer by doping a solutioncontaining 10 to 25 wt % of a hydrophilic polymer on a support; and (b)consecutively forming a polyamide layer on the hydrophilic polymer layerand then separating the support from the membrane.

In the method for preparing a forward osmosis membrane according to thepresent invention, the hydrophilic polymer constituting the hydrophilicpolymer layer includes any one selected from the group consisting ofpolyacrylonitrile, polyacrylate, polymethylmethacrylate, polyethyleneimide, cellulose acetate, cellulose triacetate, polyvinyl alcohol,polyvinylpyrrolidone, polyethyleneglycol, polysulfone-based polymer,polyethylene oxide and polyvinyl acetate or mixture thereof.

Preferably, the hydrophilic polymer as a mixture includes any oneselected from polyvinylpyrrolidone, polyvinyl alcohol,polyethyleneglycol or cellulose acetate in an amount of 0.1 to 5 wt % incombination with polyacrylonitrile.

Preferably, the hydrophilic polymer as another mixture contains 0.1 to10 wt % of a sulfonated polysulfone-based polymer represented by formula1 in combination with the polysulfone-based polymer:

where A is any one functional group selected from:

B is any one functional group selected from:

m/(n+m) is 0.2 to 0.7; and x is 50 to 2,300.

The polysulfone-based polymer is any one selected from the groupconsisting of polysulfone, polyethersulfone and polyarylethersulfone ormixture thereof. Preferably, the sulfonated polysulfone-based polymer isa compound represented by formula 2:

where m/(n+m) is 0.2 to 0.7; and x is 50 to 2,300.

In the method for preparing a forward osmosis membrane according to thepresent invention, the polyamide layer is formed by an interfacialpolymerization of an aqueous solution further containing a hydrophiliccompound and an organic solution containing a polyfunctional acyl halidecompound. Here, the aqueous solution is prepared by adding thehydrophilic compound to an aqueous solution containing polyfunctionalamine or alkylated aliphatic amine. The hydrophilic compound containsany one hydrophilic functional group selected from the group consistingof hydroxy group, sulfonate group, carbonyl group, trialkoxysilanegroup, anion group and tertiary amino group.

More preferably, the polyamide layer is formed by an interfacialpolymerization of an aqueous solution further containing a polyaminesalt compound and an organic solution containing a polyfunctional acylhalide compound. Here, the aqueous solution is prepared by adding 0.01to 2 wt % of the polyamine salt compound as an aqueous additive to anaqueous solution containing polyfunctional amine or alkylated aliphaticamine.

The polyamine salt compound is prepared from a tertiary polyamine and astrong acid at a molar ratio of 0.5˜2:1. The tertiary polyamine is anyone selected from the group consisting of 1,4-diazabicyclo[2,2,2]octane(DABCO), 1,8-diazabicyclo[5,4,0]undec-7-ene (DBU),1,5-diazabicyclo[4,3,0]none-5-ene (DBN), 1,4-dimethylpiperazine,4-[2-(dimethylamino)ethyl]morpholine,N,N,N′,N′-tetramethylethylenediamine,N,N,N′,N′-tetramethyl-1,3-butanediamine,N,N,N′,N′-tetramethyl-1,4-butanediamine (TMBD),N,N,N′,N′-tetramethyl-1,3-propanediamine,N,N,N′,N′-tetramethyl-1,6-hexanediamine (TMHD),1,1,3,3-tetramethylguanidine (TMGU) andN,N,N′,N′,N″-pentamethyldiethylenetriamine.

The aqueous solution further contains 0.01 to 2 wt % of a polar solvent,the polar solvent being any one selected from the group consisting ofethyleneglycol derivative, propyleneglycol derivative, 1,3-propanediolderivative, sulfoxide derivative, sulfone derivative, nitrilederivative, ketone derivative and urea derivative.

The forward osmosis membrane of the present invention is a compositemembrane structure having a polyamide layer sequentially laminated on ahydrophilic support layer. More specifically, the forward osmosismembrane comprises a hydrophilic polymer layer and a polyamide layer, orhas a structure of sequentially laminated layers of nonwoven fabriclayer, hydrophilic polymer layer and polyamide layer.

The hydrophilic polymer layer formed on the nonwoven fabric layer ofhigh porosity and high hydrophilicity enhances the water permeability ofthe membrane and the water flux, while the polyamide layer securescontamination resistance and chemical resistance and prevents the backdiffusion of salts of the draw solution in the direction of reverseosmosis. For that reason, the forward osmosis membrane of the presentinvention is suitable for high-concentration seawater desalination.

Moreover, the preparation method for forward osmosis membrane accordingto the present invention includes forming a hydrophilic polymer layerhaving high porosity and low pore tortoesity under optimum conditionsnot only to facilitate an inflow of water from the feed water to thedraw solution but also to secure high water permeability in thedirection of osmosis, and realizes a forward osmosis membrane withminimized contamination due to the polyamide layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a micrograph showing the cross section of a hydrophilicpolymer layer in the forward osmosis membrane according to Example 1 ofthe present invention.

FIG. 2 is a micrograph showing the cross section of a polysulfone poroussupport according to Comparative Example 1 of the present invention.

FIG. 3 is a micrograph showing the front view of a nonwoven fabric layerof the forward osmosis membrane according to Example 5 of the presentinvention.

FIG. 4 is a micrograph showing the cross section of a hydrophilicpolymer layer formed on the nonwoven fabric layer of FIG. 3.

FIG. 5 is a micrograph showing the front view of a nonwoven fabric layerof the forward osmosis membrane according to Example 11 of the presentinvention.

FIG. 6 is a micrograph showing the cross section of a hydrophilicpolymer layer formed on the nonwoven fabric layer of FIG. 5.

FIG. 7 is a micrograph showing the cross section of a hydrophilicpolymer layer containing a sulfonated polysulfone-based polymeraccording to Example 12 of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Reference will be now made in detail to the preferred embodiment of thepresent invention with reference to the attached drawings.

In accordance with a first embodiment of the present invention, there isprovided a forward osmosis membrane for seawater desalination comprisinga hydrophilic polymer layer; and a polyamide layer.

In accordance with a second embodiment of the present invention, thereis provided a forward osmosis membrane for seawater desalination havinga composite membrane structure of sequentially laminated layers thatcomprises: a nonwoven fabric layer; a hydrophilic polymer layer; and apolyamide layer.

In the structure according to the first or second embodiment, theforward osmosis membrane of the present invention is characterized by astructure provided with a hydrophilic polymer layer having high porosityand high hydrophilicity designed to facilitate the inflow of water froma feed water into a draw solution and to have high water permeability inthe direction of osmosis. As shown from the cross section of thehydrophilic polymer layer suggested in the embodiments of the presentinvention, the hydrophilic polymer layer has finger-like pores (shown inFIGS. 1, 4, 6 and 7) rather than bead-like pores as seen in theconventional reverse osmosis membrane (shown in FIG. 2). The forwardosmosis membrane of the present invention preferably satisfies a flux of3 to 30 GFD, more preferably 7 to 20 GFD with a 2M NaCl draw solution orunder the equivalent osmotic pressure condition.

The structure of the forward osmosis membrane of the present inventionis further characterized by the polyamide layer laminated on thehydrophilic polymer layer that endows the membrane with contaminationresistance and chemical resistance and particularly prevents the solutesof the draw solution from diffusing in the direction of reverse osmosisto maintain high osmotic pressure, thereby realizing a suitable membranefor separation of high-concentration seawater.

Hence, the forward osmosis membrane of the present invention maintainsexcellent flux and shows a conductivity per minute not more than 9.0μS/cm (over a membrane surface area of 24 cm²) as measured in theforward osmosis mode using a 2M NaCl solution (osmotic pressure of about100 atm) as a draw solution and ultrapure water as a feed water. Whenconverted over the membrane area (24 cm²), the back diffusion of saltsis not more than 0.375 (μS/cm)/min·cm², which shows a low salt diffusionbehavior. The back diffusion of salts means that the solutes of thehigher concentration draw solution are transported into the lowerconcentration feed water. Hence, the forward osmosis membrane of thepresent invention is suitable for separation of high-concentrationseawater. Contrarily, if the conductivity is more than 9.0 μS/cm perminute over a membrane surface area of 24 cm² as measured in the forwardosmosis mode, a large quantity of salts contained in the draw solutionflows into the feed water, increases the back diffusion of salts andthus deteriorates the performance of the membrane. Furthermore, a lossof salts in the draw solution results in a decreased osmotic pressure,causing not only a sudden drop of the permeation flux but also a needfor continuously supplying salts in the draw solution in order tomaintain the osmotic pressure at a constant level. Accordingly, theseawater desalination effect can be achieved when the membrane has aconductivity not more than 9.0 μS/cm per minute over a membrane surfacearea of 24 cm², or when converted over the membrane area (24 cm²), aback diffusion of salts is not more than 0.375 (μS/cm)/min·cm².

The polyamide layer can even remove monovalent ions that are hard toremove with a single-structure membrane, so the forward osmosis membranehaving the polyamide layer of the present invention secures more than90% salt rejection rate.

Hereinafter, the forward osmosis membrane of the present invention willbe described in detail component by component.

1) Nonwoven Fabric Layer

In the forward osmosis membrane of the present invention, the nonwovenfabric layer acts as a support of the membrane.

The preferred material used for the nonwoven fabric layer of the presentinvention is synthetic fabric selected from the group consisting ofpolyester, polypropylene, nylon and polyethylene; or natural fabricincluding cellulose. According to the porosity and hydrophilicity of itsmaterial, the nonwoven fabric layer determines the properties of themembrane.

Preferably, the nonwoven fabric layer may include, but is notspecifically limited to, any material having a porosity to meet an airpermeability at least 2 cc/cm²·sec, more preferably 2 to 20 cc/cm²·sec.The average pore diameter of the nonwoven fabric layer of the presentinvention is preferably 1 to 600 μm, more preferably 5 to 300 μm, inwhich the membrane facilitates an inflow of water and shows higher waterpermeability as necessary for the forward osmosis membrane.

FIG. 3 is a micrograph showing the surface of a nonwoven fabric webhaving an air permeability of 2 to 10 cc/cm²·sec as used in anembodiment of the present invention. FIG. 5 is a micrograph showing thesurface of a nonwoven fabric web having an air permeability of at least10 cc/cm²·sec in an embodiment of the present invention. The higherporosity of the nonwoven fabric layer more facilitates an inflow ofwater from the feed water into the draw solution and secures higherwater permeability in the direction of osmosis.

Contrarily, FIG. 2 is a micrograph showing the cross section of apolysulfone porous support used in a conventional reverse osmosismembrane, which has a denser structure in comparison with the porosityas seen in FIGS. 3 and 5.

The nonwoven fabric used in the embodiment of the present invention hassuch a high hydrophilicity as to absorb water within 5 seconds at lessthan 5 degrees of contact angle immediately after getting in contactwith water, while the nonwoven fabric used for a reverse osmosismembrane shows a contact angle of 75 to 90 degrees. The material usefulfor the nonwoven fabric layer of the present invention preferably hashydrophilicity less than 0.1 to 75 degrees and, more preferably, acontact angle of 0.1 to 60 degrees. Hence, the nonwoven fabric layer ofthe present invention meets high hydrophilicity, it is required toreduce water resistance and to prevent contamination of the membranecaused by internal concentration polarization (ICP). The internalconcentration polarization (ICP) deteriorates the permeability of themembrane due to contamination in the membrane and significantly reducesthe flux particularly in a forward osmosis membrane that is driven bythe osmotic pressure resulting from a concentration difference naturallyoccurring.

The thickness of the nonwoven fabric layer of the present invention ispreferably in the range of 20 to 150 μm. The nonwoven fabric layer lessthan 20 μm thick is too weak to support the whole membrane, while thenonwoven fabric layer more than 150 μm thick causes a deterioration ofthe flux.

2) Hydrophilic Polymer Layer

In the forward osmosis membrane of the present invention, a hydrophilicmaterial is used for the hydrophilic polymer layer in order to minimizewater-aided permeation resistance.

Preferably, the hydrophilic polymer layer includes any one selected fromthe group consisting of polyacrylonitrile, polyacrylate,polymethylmethacrylate, polyethylene imide, cellulose acetate, cellulosetriacetate, polyvinyl alcohol, polyvinylpyrrolidone, polyethyleneglycol,polysulfone-based polymer, polyethylene oxide and polyvinyl acetate ormixture thereof.

More preferably, the hydrophilic polymer as a mixture form contains anyone selected from polyvinylpyrrolidone, polyvinyl alcohol,polyethyleneglycol or cellulose acetate in an amount of 0.1 to 5 wt % incombination with polyacrylonitrile. The mixing ratio less than 0.1 wt %hardly realizes a property improving effect of the polymer added, whilethe mixing ratio greater than 5 wt % leads to an excessively highviscosity of the hydrophilic polymer solution, making it difficult toprepare the support layer.

The hydrophilic polymer as another mixture form contains a syntheticpolymer prepared by copolymerization of polyacrylonitrile (PAN) and apolymer having a hydrophilic functional group. The polymer havinghydrophilic a functional group is a polymer compatible withpolyacrylonitrile (PAN). The hydrophilic functional group of the polymeris selected from hydroxyl group, sulfonate group, carbonyl group,acetate group, or ester group. The preferred examples of the syntheticpolymer are PAN-vinyl acetate copolymer, or PAN-acrylic ester copolymer.

When the hydrophilic polymer is treated with base (OH), the membrane hasenhanced hydrophilicity. Hence, the hydrophilic polymer used in thepresent invention includes a compound treated to hydrophilicity.

The hydrophilic polymer layer of the present invention is formed by ahydrophilic polymer mixture comprising 0.1 to 10 wt % of a sulfonatedpolysulfone-based polymer represented by the following formula 1 incombination with the polysulfone-based polymer:

where A is any one functional group selected from:

B is any one functional group selected from:

m/(n+m) is 0.2 to 0.7; and x is 50 to 2,300.

In the embodiment of the present invention, a preferred example of thesulfonated polysulfone-based polymer may include, but is not limited to,a compound represented by the following formula 2:

where m/(n+m) is 0.2 to 0.7; and x is 50 to 2,300.

The preferred example of the polysulfone-based polymer includes any oneselected from the group consisting of polysulfone, polyethersulfone andpolyarylethersulfone, or mixture thereof.

Preferably, the mixture form contains 0.1 to 10 wt % of a sulfonatedpolysulfone-based polymer represented by the formula 1 in combinationwith polysulfone. The mixture ratio less than 0.1 wt % hardly realizesthe hydrophilic effect of the polymer added, while the mixture ratioexceeding 10 wt % leads to an excessively high viscosity of thesolution, consequently with difficulty in preparing the solution. Thus,the membrane is difficult to making the polyamide layer due to anextreme increase of the hydrophilicity of the polymer layer.

In accordance with the hydrophilic polymer layer formed by using asulfonated polysulfone-based polymer as a hydrophilic material incombination with the polysulfone-based polymer, the hydrophilic polymerlayer is minimizing the permeation resistance of water. FIG. 7 is amicrograph showing a cross section of the hydrophilic polymer layercontaining the sulfonated polysulfone-based polymer, in which thehydrophilic polymer layer shows high porosity and low pore tortuositydue to a uniform finger-like pore structure. The porosity dependent onthe hydrophilicity can be controlled by the choice of the hydrophilicpolymer.

The thickness of the hydrophilic polymer layer is desirably minimized inorder to increase the flux, preferably in the range of 30 to 250 μm.

2) Polyamide Layer

In the forward osmosis membrane of the present invention, the polyamidelayer is formed by an interfacial polymerization of an aqueous solutioncontaining polyfunctional amine or alkylated aliphatic amine and anorganic solution containing a polyfunctional acyl halide compound on thehydrophilic polymer.

More specifically, on the surface of the hydrophilic polymer layerformed on the nonwoven fabric layer, the polyamide layer is formed by aninterfacial polymerization of an aqueous solution containingpolyfunctional amine or alkylated aliphatic amine selected fromm-phenyldiamine, p-phenyldiamine, o-phenyldiamine, piperazine, oralkylated piperazine in contact with an organic solution containing apolyfunctional acyl halide compound selected from polyfunctionalsulfonyl halide or polyfunctional isocyanate.

Otherwise, a hydrophilic compound is further added to the aqueoussolution containing polyfunctional amine or alkylated aliphatic amine.The resulting aqueous solution is then put in contact with an organicsolution containing a poly multi-functional acyl halide compound on thesurface of the hydrophilic polymer layer, causing an interfacialpolymerization reaction between the compounds to form a polyamide layerwith enhanced contamination resistance. Preferably, the hydrophiliccompound is contained in the aqueous solution in an amount of 0.001 to 8wt %, more preferably 0.01 to 4 wt %.

Preferably, the hydrophilic compound added to the aqueous solutioncontaining polyfunctional amine or alkylated aliphatic amine is ahydrophilic compound having at least one hydrophilic functional groupselected from the group consisting of hydroxyl group, sulfonate group,carbonyl group, trialkoxysilane group, anion group, or tertiary aminogroup. More preferably, the hydrophilic compound is a hydrophilic aminocompound.

More preferably, the preferred example of the hydrophilic compoundhaving a hydroxyl group is selected from the group consisting of1,3-diamino-2-propanol, ethanolamine, diethanolamine,3-amino-1-propanol, 4-amino-1-butanol and 2-amino-1-butanol.

The hydrophilic compound having a carbonyl group is selected from thegroup consisting of amino-acetaldehyde dimethyl acetal,α-aminobutyrolactone, 3-aminobenzamide, 4-aminobenzamide, andN-(3-aminopropyl)-2-pyrrolidinone.

The hydrophilic compound having a trialkoxysilane group is selected fromthe group consisting of (3-aminopropyl)triethoxysilane and(3-aminopropyl)trimethoxysilane.

The hydrophilic compound having an anion group is selected from thegroup consisting of glycine, taurine, 3-amino-1-propenesulfonic acid,4-amino-1-butenesulfonic acid, 2-aminoethylhydrogene sulfate,3-aminobenzenesulfonic acid, 3-amino-4-hydroxybenzenesulfonic acid,4-aminobenzenesulfonic acid, 3-aminopropylphosphonic acid,3-amino-4-hydroxybenzoic acid, 4-amino-3-hydroxybenzoic acid,6-aminohexeneoic acid, 3-aminobutaneoic acid, 4-amino-2-hydroxybutyricacid, 4-aminobutyric acid and glutamic acid.

The hydrophilic compound having at least one tertiary amino group isselected from the group consisting of 3-(diethylamino)propylamine,4-(2-aminoethyl)morpholine, 1-(2-aminoethyl)piperazine,3,3′-diamino-N-methyldipropylamine and 1-(3-aminopropyl)imidazole.

More preferably, the polyamide layer of the present invention is formedby an interfacial polymerization of an aqueous solution furthercontaining a polyamine salt compound in contact with an organic solutioncontaining a polyfunctional acyl halide compound. The polyamide layer isformed on polymer layer, which laminated on the support consisting ofthe nonwoven fabric layer. Here, the aqueous solution is prepared byfurther adding 0.01 to 2 wt % of the polyamine salt compound as anaqueous additive to an aqueous solution containing polyfunctional amineor alkylated aliphatic amine.

An addition of the polyamine salt compound to the polyfunctionalamine-containing aqueous solution is beneficial in formation of pores ofthe polyamide layer, so the pores acting as acid acceptors enhance fluxand accelerate the interfacial reaction. The polyamine salt compound ispreferably used in an amount of 0.01 to 2 wt %. The content of thepolyamine salt compound less than 0.01 wt % hardly realizes formation ofpores for enhancing the flux, while the content of the polyamine saltcompound exceeding 2 wt % affects the formation of a polyamide chain tocause defectives in the coating layer.

More preferably, the polyamine salt is a tertiary polyamine saltcompound prepared from a tertiary polyamine and a strong acid at a molarratio of 0.5˜2:1. The molar ratio of the tertiary polyamine salt lessthan 0.5 hardly realizes the effect of the tertiary polyamine salt forenhancing the flux, while the molar ratio of the tertiary polyamine saltexceeding 2 allows polyamine remaining after the reaction to affect theformation of the polyamide chain.

The examples of the strong acid include any one selected from the groupconsisting of aromatic sulfonic acid, aliphatic sulfonic acid,cycloaliphatic sulfonic acid, trifluoroacetic acid, nitric acid,hydrochloric acid and sulfonic acid or mixture thereof.

The polyamine used in the present invention is a compound that at leasttwo monovalent or divalent amines bond together. The polyamine ispreferably an aliphatic or aromatic polyamine, more preferably atertiary polyamine. The tertiary polyamine includes any one selectedfrom the group consisting of 1,4-diazabicyclo[2,2,2]octane (DABCO),1,8-diazabicyclo[5,4,0]undec-7-ene (DBU),1,5-diazabicyclo[4,3,0]none-5-ene (DBN), 1,4-dimethylpiperazine,4-[2-(dimethylamino)ethyl]morpholine,N,N,N′,N′-tetramethylethylenediamine,N,N,N′,N′-tetramethyl-1,3-butanediamine,N,N,N′,N′-tetramethyl-1,4-butanediamine (TMBD),N,N,N′,N′-tetramethyl-1,3-propanediamine,N,N,N′,N′-tetramethyl-1,6-hexanediamine (TMHD),1,1,3,3-tetramethylguanidine (TMGU) andN,N,N′,N′,N″-pentamethyldiethylenetriamine.

For the purpose of improving the flux of the membrane, the aqueoussolution of polyfunctional amine may further contain 0.01 to 2 wt % ofat least one or two polar solvents as well as the polyamine saltcompound. The polar solvent is selected from the group consisting ofethyleneglycol derivative, propyleneglycol derivative, 1,3-propanediolderivative, sulfoxide derivative, sulfone derivative, nitrilederivative, ketone derivative and urea derivative, or mixture thereof.

As describe above, the forward osmosis membrane of the present inventionnot only has the polyamide layer secure high salt rejection, chemicalresistance and pH stability but also further contains the polyamine saltcompound added as an aqueous additive in forming the polyamide layer toenhance the flux of the membrane and to prevent the solutes of the drawsolution from diffusing in the direction of reverse osmosis.

The forward osmosis membrane of the present invention can even removemonovalent ions that are hard to remove with a single membranestructure, thereby securing high salt rejection.

The present invention provides a method for preparing the forwardosmosis membrane for seawater desalination according to the presentinvention.

More specifically, there is provided a method for preparing a forwardosmosis membrane for seawater desalination having a three-layeredstructure of nonwoven fabric layer, hydrophilic polymer layer andpolyamide layer that includes: (a) forming a hydrophilic polymer layerby doping a solution containing 10 to 25 wt % of a hydrophilic polymeron a nonwoven fabric layer; and (b) forming a polyamide layer by aninterfacial polymerization reaction of an organic solution containing apolyfunctional acyl halide compound in contact with an aqueous solutioncontaining polyfunctional amine or alkylated aliphatic amine on thehydrophilic polymer layer.

There is also provided a method for preparing a forward osmosis membranefor seawater desalination having a two two-layered structure ofhydrophilic polymer layer and polyamide layer that includes: (a) forminga hydrophilic polymer layer by doping a solution containing 10 to 25 wt% of a hydrophilic polymer on a support having a smooth surface, such asglass plate or nonwoven fabric; and (b) consecutively forming apolyamide layer on the hydrophilic polymer layer and then separating thesupport from the membrane.

The step (a) is designed for the forward osmosis membrane to facilitatean inflow of water from the feed water into the draw solution and tosecure high water permeability in the direction of osmosis.

To achieve the designing purpose of the step (a), the hydrophilicpolymer layer is formed using a solution containing a hydrophilicpolymer.

The hydrophilic polymer layer uses a hydrophilic material to facilitatewater flow and secure high water permeability in the direction ofosmosis. The preferred examples of the hydrophilic polymer include anyone selected from the group consisting of polyacrylonitrile,polyacrylate, polymethylmethacrylate, polyethylene imide, celluloseacetate, cellulose triacetate, polyvinyl alcohol, polyvinylpyrrolidone,polyethyleneglycol, polysulfone-based polymer, polyethylene oxide andpolyvinyl acetate or mixture thereof.

The example of the hydrophilic polymer also includes a mixture form incombination with the hydrophilic polymer, or a synthetic copolymer ofthe hydrophilic polymer and a hydrophilic compound having a hydrophilicfunctional group.

Another example of the hydrophilic polymer includes a compound endowedwith hydrophilicity through a base-treatment for increasinghydrophilicity.

The specific example of the hydrophilic polymer is the same aspreviously stated in regard to the forward osmosis membrane and will notbe described in further detail.

In the step (a), the content of the hydrophilic polymer in thehydrophilic polymer containing solution is preferably in the range of 10to 25 wt %, more preferably 13 to 20 wt %. The hydrophilic polymercontent less than 10 wt % tends to produce large-sized pores, withfailure to form a porous structure capable of removing salts andconsequently a deterioration of membrane separation performance. Thehydrophilic polymer content of more than 25 wt % increases viscosity,making membrane formation difficult, or produces less or too minutepores, deteriorating the membrane separation performance.

The hydrophilic polymer layer of the present invention tends to havehigher porosity with an increase in the hydrophilicity that is dependenton the type and content of the hydrophilic polymer, and the pores thusobtained have a uniform finger-like structure and consequently low poretortuosity (see FIGS. 1, 4, 6 and 7).

The thickness of the hydrophilic polymer layer is preferably minimizedto 30 to 250 μm in order to increase the flux.

In the step (a), the nonwoven fabric layer is provided not only to actas a support for the membrane but also to facilitate water flowpertaining to coarse porosity and high hydrophilicity.

Preferably, the material useful for the nonwoven fabric layer mayinclude, but is not limited to, any material having porosity that allowsan air permeability at least 2 cc/cm²·sec. More preferably, the materialfor the nonwoven fabric layer secures an air permeability in the rangeof 2 to 20 cc/cm²·sec. The nonwoven fabric can be a nonwoven fabricprepared by, if not specifically limited to, the conventionalpreparation method, more preferably a wet nonwoven fabric prepared bythe paper making process. The nonwoven fabric layer may be included inor detached from the final forward osmosis membrane structure.

In the preparation method of the present invention, the step (b) is toform a polyamide layer on the hydrophilic polymer layer prepared in thestep (a).

The polyamide layer is prepared by an interfacial polymerization of anorganic solution containing a polyfunctional acyl halide compoundselected from polyfunctional acyl halide, polyfunctional sulfonylhalide, or polyfunctional isocyanate and an aqueous solution containingpolyfunctional amine or alkylated aliphatic amine selected fromm-phenyldiamine, p-phenyldiamine, o-phenyldiamine, or alkylatedpiperidine.

In the preparation method of the present invention, the polyamide layeris also prepared by an interfacial polymerization of an aqueous solutionfurther containing a hydrophilic compound and an organic solutioncontaining polyfunctional acyl halide compound, the aqueous solutionbeing prepared by adding the hydrophilic compound to an aqueous solutioncontaining polyfunctional amine or alkylated aliphatic amine, thehydrophilic compound having any one hydrophilic functional groupselected from the group consisting of hydroxy group, sulfonate group,carbonyl group, trialkoxysilane group, anion group and tertiary aminogroup.

More preferably, the polyamide layer is prepared by an interfacialpolymerization of an aqueous solution further containing a polyaminesalt compound and an organic solution containing a polyfunctional acylhalide compound, the aqueous solution further containing a polyaminesalt compound being prepared by adding 0.01 to 2 wt % of the polyaminesalt compound as an aqueous additive to an aqueous solution containingpolyfunctional amine or alkylated aliphatic amine.

The polyamine salt compound is prepared from a tertiary polyamine and astrong acid at a molar ratio of 0.5˜2:1. The tertiary polyamine is anyone selected from the group consisting of 1,4-diazabicyclo[2,2,2]octane(DABCO), 1,8-diazabicyclo[5,4,0]undec-7-ene (DBU),1,5-diazabicyclo[4,3,0]none-5-ene (DBN), 1,4-dimethylpiperazine,4-[2-(dimethylamino)ethyl]morpholine,N,N,N′,N′-tetramethylethylenediamine,N,N,N′,N′-tetramethyl-1,3-butanediamine,N,N,N′,N′-tetramethyl-1,4-butanediamine (TMBD),N,N,N′,N′-tetramethyl-1,3-propanediamine,N,N,N′,N′-tetramethyl-1,6-hexanediamine (TMHD),1,1,3,3-tetramethylguanidine (TMGU) andN,N,N′,N′,N″-pentamethyldiethylenetriamine.

For the purpose of increasing the flux of the membrane, at least one ortwo polar solvents as well as the polyamine salt compound is furtheradded to the polyfunctional amine aqueous solution in an amount of 0.01to 2 wt %. The polar solvent as used herein is selected from the groupconsisting of ethyleneglycol derivative, propyleneglycol derivative,1,3-propanediol derivative, sulfoxide derivative, sulfone derivative,nitrile derivative, ketone derivative and urea derivative.

In the preparation method of the present invention, a composite membraneof laminated layers is completed in step (b) that has the polyamidelayer laminated on the hydrophilic polymer layer acting as a poroussupport. The polyamide layer secures contamination resistance andchemical resistance. Contrary to a single layer structure that hardlyremoves minute salts such as monovalent ions because of large pore size,the composite membrane structure of the present invention having thepolyamide layer can remove even monovalent ions, that is, achieving highsalt rejection and preventing the back diffusion of the solutes of thedraw solution in the direction of reverse osmosis.

In the preparation method for the forward osmosis membrane of thepresent invention, in order to provide a stable layer structure, thecomplete membrane is formed to include a hydrophilic polymer layer incombination with a nonwoven fabric layer formed as a support, and apolyamide layer consecutively formed on an interfacial surface of thehydrophilic polymer layer.

In an alternative manner, the nonwoven fabric layer can be detached fromthe whole membrane structure. The process of separating the support fromthe membrane is well known to those skilled in the art and will not befurther described in this specification.

The support as used herein is any material having a smooth surface andpreferably includes, but is not limited to, glass plate, nonwovenfabric, or the like.

Hereinafter, the present invention will be described in detail inconnection with preferred examples.

It is obvious that the examples are not intended to limit scope of theinvention to those examples.

1. Preparation of Forward Osmosis Membrane for Seawater DesalinationHaving Two-Layered Structure Example 1

A solution containing 17.5 wt % of polyacrylonitrile as a hydrophilicpolymer in an organic solvent was cast 50 μm thick on a glass plate andthen subjected to phase transition in water used as a nonsolvent at theroom temperature to form a hydrophilic polymer layer. The hydrophilicpolymer layer thus obtained was kept in ultrapure water for about oneday to extract the solvent. On the surface of the solvent-extractedmembrane, an aqueous solution containing 2 wt % of m-phenylenediamine(MPD) was put in interfacial contact with an organic solution containing0.1 wt % of trimesoyl chloride (TMC) in an ISOPAR solvent (Exxon Corp.)to form a polyamide layer by a polymerization reaction between thecompounds, completing a composite membrane.

Examples 2, 3 and 4

The procedures were performed to prepare a composite membrane in thesame manner as described in Example 1, excepting that the membranecomposition was given as presented in Table 1.

Comparative Example 1

The procedures were performed to prepare a composite membrane in thesame manner as described in Example 1, excepting that a polyamide layeris formed on a porous polysulfone support.

Comparative Example 2

A membrane was prepared that consists of a single cellulose triacetatematerial.

Experimental Example 1 Flux Measurement

On both sides of the membrane prepared above, water was induced to flowfrom the feed water to the draw solution. The before and after weight ofthe draw solution over time was measured to determine the quantity ofwater per time. Here, the draw solution was 2M NaCl solution, and thefeed water was ultrapure water (the osmotic pressure about 100 atm).

Experimental Example 2 Measurement of Back Diffusion of Salts

The membranes prepared above were measured in regard to a change of theelectrical conductivity of salts flowing from the draw solution to thefeed water with a conductivity meter, where the feed water was ultrapurewater (the osmotic pressure about 100 atm) and the draw solution wassalt water (2M NaCl). The back diffusion degree of salts was used toevaluate the unit of conductivity variation (μS/cm) per minute over adefined membrane area (24 cm²) [the conductivity of solids dissolved inwater: μS/cm×0.5˜0.6=TDS (Total Dissolved Solids, mg/L)].

The conductivity per minute [(μS/cm)/min] over the membrane area (24cm²) was converted a presented in Table 1. The back diffusion degree ofsalts was evaluated from the results.

Experimental Example 3 Measurement of Salt Rejection

The salt rejection of the membrane prepared above was determined bymeasuring the amount of salt in the used feed water and the amount ofsalt contained in the water passing through the membrane to evaluate thesalt ratio of the passed water to the feed water in percentage. Here,the amount of salts in the feed water and the passed water was measuredby IC, and the salt rejection of the membrane was determined as a saltratio of the passed water to the feed water.

As presented in Table 1, the property measurement results of themembrane were evaluated in the forward osmosis mode under the solutioncondition of 2M NaCl solution/ultrapure water.

TABLE 1 Properly Assessment of Membrane Using 2M NaCl Draw SolutionMembrane Composition Change of the (Polymer Conductivity Back Diffusionof Salt Layer/Polyamide Flux per min Salts per Area Rejection Div.Layer) (GFD) ((μS/cm)/min) ((μS/cm)/min · cm²) (%) Example 1 17.5 wt %18.2 3.8 0.158 97.8 PAN/PA Example 2 15 wt % P-CO- 20.6 4.2 0.175 97.9PAN/PA Example 3 17 wt % PSf + 1 16.0 6.3 0.263 95.9 wt % S-PSf/PAExample 4 19 wt % PSf + 1 16.4 0.6 0.0250 99.6 wt % S-PSf/PA Comparative18 wt % PSf/—/ 1.2 0.065 0.00271 99.4 Example 1 PA Comparative CTA 109.12 0.380 90.4 Example 2 Effective Membrane Area: 24 cm² PAN:Polyacrylonitrile P-CO-PAN: Polyacrylonitrile-vinylacetate copolymer PA:Polyamide PSf: Polysulfone S-PSf: Sulfonated Polysulfone CTA: CelluloseTriacetate

As shown in Table 1, when the membrane of Comparative Example 1 having apolyamide layer on a polysulfone porous support prepared as aconventional reverse osmosis membrane was put in the forward osmosismode, the back diffusion of salts appeared considerably insignificantwith a great drop of flux. In the membrane structure of ComparativeExample 1, the formation of the polyamide layer is minimized the backdiffusion of salts.

The membrane of Comparative Example 2 that has a single membranestructure of a single cellulose triacetate material as a hydrophilicmaterial showed considerably high flux but had a great variation of theback diffusion of salts, which makes the membrane impractical when usedas a forward osmosis membrane.

Whereas, the membranes prepared in the Examples of the present inventionwere controllable in porosity and hydrophilicity depending on the typeand content of a polymer constituting the hydrophilic polymer material.More specifically, the membranes of the present invention had excellentflux, that is, at least 16.0 GFD, and low conductivity less than 9.0(μS/cm)-per minute (over a membrane area of 24 cm²), showing asatisfactory behavior regarding the back diffusion of salts andovercoming the problem of allowing the solutes of the draw solution todiffuse in the direction of reverse osmosis.

FIG. 1 is a 700× micrograph showing the cross section of a hydrophilicpolymer layer in the forward osmosis membrane according to Example 1.The hydrophilic polymer layer had high porosity and low tortuosity dueto a uniform finger-like pore structure. FIG. 2 is a micrograph showingthe cross section of a polysulfone porous support used for aconventional reverse osmosis membrane, which shows a dense structurewith respect to the porosity of the membrane prepared in Example 1.

2. Preparation of Forward Osmosis Membrane for Seawater DesalinationHaving Three-Layered Structure Example 5

A solution containing 17 wt % of polyacrylonitrile as a hydrophilicpolymer in an organic solvent was cast 50 μm thick on a nonwoven fabric1 having an air permeability of 6.3 cc/cm²·sec as prepared by the papermaking process, and then subjected to phase transition in water used asa nonsolvent at the room temperature to form a hydrophilic polymerlayer. The nonwoven fabric 1 had a contact angle changed from 80 degreesto one degree over 4 seconds, with an average pore diameter of 7.5 μm.The hydrophilic polymer layer formed on the nonwoven fabric layer waskept in ultrapure water for about one day to extract the solvent. On thesurface of the solvent-extracted membrane, an aqueous solutioncontaining 2 wt % of m-phenylenediamine (MPD) was put in interfacialcontact with an organic solution containing 0.1 wt % of trimesoylchloride (TMC) in an ISOPAR solvent (Exxon Corp.) to form a polyamidelayer by a polymerization reaction between the compounds, completing acomposite membrane.

FIG. 3 is a 700× SEM (Scanning Electron Microscope) micrograph showingthe front view of the nonwoven fabric layer in the composite membrane.FIG. 4 is a 700× micrograph showing the side view of the hydrophilicpolymer layer in the composite layer. As can be seen from FIGS. 3 and 4,the nonwoven fabric layer and the hydrophilic polymer layer constitutingthe composite membrane of Example 1 had high porosity and low poretortuosity because of a uniform finger-like pore structure.

Examples 6 to 10

The procedures were performed to prepare a composite membrane in thesame manner as described in Example 5, excepting that the membranecomposition was given as presented in Table 2.

Example 11

The procedures were performed to prepare a composite membrane in thesame manner as described in Example 5, excepting that there was usednonwoven fabric 2 with coarse porosity having an air permeability atleast 10 cc/cm²·sec as shown in FIG. 5. The nonwoven fabric 2 had acontact angle changed from 114 degrees to 4 degrees over one second,with an average pore diameter of 293 μm. FIG. 6 is a micrograph showingthe side view of a hydrophilic polymer layer formed on the nonwovenfabric 2 of FIG. 5.

The membranes prepared in Examples 5 to 11 were measured in regard toflux and back diffusion of the salts in the same manner as described inExperimental Examples 1 and 2. The results are presented in Table 2.

TABLE 2 Property Assessment of Membrane Using 2M NaCl Draw SolutionChange of Draw the Solution Conductivity Back Diffusion of MembraneComposition Flux per mm Salts per Area Div. Composition (DS/FS) (GFD)(μS/cm)/min ((μS/cm)/min · cm²) Example 5 Nonwoven 2M NaCl/DI 4.16 0.390.0163 Fabric 1/ water 17 wt % PAN/PA Example 6 Nonwoven 2M NaCl/DI 3.170.15 0.00625 Fabric 1/ water 13 wt % PAN/PA Example 7 Nonwoven 2MNaCl/DI 3.73 0.95 0.396 Fabric 1/ water 10 wt % PAN/PA Example 8Nonwoven 2M NaCl/DI 3.35 0.30 0.0125 Fabric 1/ water 17 wt % CO- PAN/PAExample 9 Nonwoven 2M NaCl/DI 2.99 0.41 0.0171 Fabric 1/ water 20 wt %CO- PAN/PA Example 10 Nonwoven 2M NaCl/DI 3.32 0.093 0.00388 Fabric 1/water 20 wt % P- CO-PAN/PA Example 11 Nonwoven 2M NaCl/DI 8.6 1.5 0.0625Fabric 2/ water 17 wt % 2M NaCl/DI 9.75 1.8 0.0750 PAN/PA water(reverse) Comparative 18% PSF/—/ 2M NaCl/DI 1.2 0.065 0.00271 Example 1PA water Comparative CTA 2M NaCl/DI 10 9.12 0.380 Example 2 waterNonwoven fabric 1: air permeability of 2 to 10 cc/cm² · sec Nonwovenfabric 2: air permeability of at least 10 cc/cm² · sec PAN:Polyacrylonitrile P-CO-PAN: Polyacrylonitrile-vinylacetate copolymerCO-PAN: Polyacrylonitrile-acrylic ester copolymer PA: Polyamide PSF:Polysulfone S-PSf: Sulfonated Polysulfone CTA: Cellulose Triacetate

As shown in Table 2, the membranes prepared in the examples of thepresent invention were controllable in porosity and hydrophilicitydepending on the type of nonwoven fabric or the type and content of thepolymer constituting the hydrophilic polymer layer. The membranes of thepresent invention showed excellent flux and a conductivity not more than1.8 μS/cm per minute (over a membrane area of 24 cm²), or a change ofconductivity per area not more than 0.075 (μS/cm)/min·cm², therebyrealizing a forward osmosis membrane preventing solutes of the drawsolution from diffusing in the direction of reverse osmosis.

As shown in Examples 5 and 11, with the higher porosity of the nonwovenfabric layer in the same composition membrane (nonwoven fabric1<nonwoven fabric 2), the flux was considerably enhanced to the level ofComparative Example 2, but the back diffusion of salts was considerablylow relative to Comparative Example 2. These results show that theforward osmosis membrane of the present invention not only maintainshigh flux but also exhibits low back diffusion of salts, preventingsolutes of the draw solution from diffusing in the direction of reverseosmosis.

Example 12 1. Preparation of Sulfonated Sulfone-Based Polymer

5 g of polysulfone and 50 mL of dichloromethane as monomers were put ina 100 mL four-necked flask equipped with a mechanical stirrer and anitrogen inlet. The reaction solution was stirred at the roomtemperature for 18 hours and cooled down to 20° C. 0.5 mL ofchlorosulfone diluted with dichloromethane was gradually added to thesolution. The solution was stirred for 5 hours to form a copolymer.After completion of the reaction, the solution was removed of theresidual solvent, and the precipitate was neutralized with a neutralizer(1N NaOH). After neutralization, the precipitate was washed andsubjected to filtration. The copolymer product thus obtained was driedout in a vacuum oven at 80° C. for 24 hours to yield a sulfonatedpolysulfone-based polymer represented by the following formula 2 (weightaverage molecular weight: 69,000; and 50% sulfonation degree).

2. Preparation of Forward Osmosis Membrane

A hydrophilic polymer solution containing 17 wt % of polysulfone and 1wt % of the sulfonated polysulfone-based polymer prepared in theprecedent step was cast 150 μm thick on a nonwoven fabric layer havingporosity with an air permeability of at least 6.3 cc/cm²·sec. Thesolution was subjected to phase separation in water used as a nonsolventat the room temperature to form a hydrophilic polymer support. Thehydrophilic polymer support formed on the nonwoven fabric layer was keptin ultrapure water for about one day to extract the solvent. On thesurface of the solvent-extracted membrane, an aqueous solutioncontaining 2 wt % of m-phenylenediamine (MPD) was put in interfacialcontact with an organic solution containing 0.1 wt % of trimesoylchloride (TMC) in an ISOPAR solvent (Exxon Corp.) to form a polyamidelayer by a polymerization reaction between the compounds, completing acomposite membrane.

Example 13

The procedures were performed to prepare a membrane in the same manneras described in Example 12, excepting that there was used a hydrophilicpolymer solution containing 18 wt % of polysulfone and 1 wt % ofsulfonated polysulfone, as given in Table 3.

The membranes prepared in Examples 12 and 13 were measured in regard toflux and back diffusion of salts in the same manner as described inExperimental Examples 1 and 2. The results are presented in Table 3.

TABLE 3 Property Assessment of Membrane Using 2M NaCl Draw Solution DrawSolution Change of the Back Diffusion of Membrane Composition FluxConductivity Salts per Area Div. Composition (DS/FS) (GFD) (μS/cm)/min((μS/cm)/min · cm²) Example 12 Nonwoven 2M NaCl/DI 10.10 1.54 0.0642Fabric 2/ water 17 wt % PSF + 1 wt % S-PSF/PA Example 13 Nonwoven 2MNaCl/DI 11.24 0.36 0.0150 Fabric 2/ 2M NaCl/DI 13.75 3.81 0.159 18 wt %(Reverse) PSF + 1 wt % S-PSF/PA Comparative 18 wt % 2M NaCl/DI 1.2 0.0650.00271 Example 1 PSF/—/PA Comparative CTA 2M NaCl/DI 10 9.12 0.380Example 2 Nonwoven fabric 2: air permeability at least 10 cc/cm² · secPA: Polyamide PSF: Polysulfone S-PSF: Sulfonated Polysulfone CTA:Cellulose Triacetate DI: Ultrapure Water

As shown in Table 3, the membranes prepared in Examples 12 and 13exhibited excellent flux and a change of conductivity in the range of0.36 to 1.54 μS/cm per minute (over a membrane area of 24 cm²),realizing low back diffusion of salts. The back diffusion of the saltswas evaluated to 0.0150 to 0.0642 (μS/cm)/min·cm². Thus, the membranesof Examples 12 and 13 met the requirement as a forward osmosis membranethat the solutes of the draw solution were prevented from diffusing inthe direction of reverse osmosis. Besides, the membranes werecontrollable in porosity and hydrophilicity depending on the type andcontent of the polymer constituting the polymer support. Morespecifically, as shown in Examples 12 and 13, the flux was considerablyincreased depending on the polymer composition or the content of thesulfonated polysulfone used as a hydrophilic polymer.

In other words, the flux was remarkably enhanced to the extent as seenfrom the membrane consisting of a hydrophilic polymer in ComparativeExample 2. The flux was more improved with an increase in thehydrophilic polymer content dependent on the sulfonated polysulfonecontent, and there existed an appropriate concentration with respect tothe hydrophilic polymer content. In this manner, the forward osmosismembrane of the present invention not only maintains high flux but alsoprevents a back diffusion of solutes from the draw solution.

The membrane of Example 13 was measured in regard to flux variation byinducing the flow of the feed water in the direction of the ForwardOsmosis mode or the Pressure Retarded Osmosis mode. The results showedthat the flux was more enhanced in the PRO mode that the feed waterflowed from the nonwoven fabric layer to the polyamide layer, ratherthan the FO mode that the feed water flowed from the polyamide layer tothe nonwoven fabric layer.

FIG. 7 is a micrograph showing a cross section of the forward osmosismembrane having a hydrophilic polymer support layer containing asulfonated polysulfone-based polymer in combination with a polysulfonepolymer. The hydrophilic polymer layer was endowed with high porositydue to a uniform finger-like pore structure. The hydrophilic polymerlayer also had low pore tortuosity due to the uniform finger-like porestructure. As seen from the examples of the present invention, thehydrophilic polymer support layer of the present invention iscontrollable in porosity by regulating the hydrophilic polymer content.

Example 14

A dimethylformamide solution containing 17.5 wt % of polyacrylonitrile(PAN) was cast 50±10 μm thick on a nonwoven polyester fabric (nonwovenfabric 1) having porosity with an air permeability of 6.3 cc/cm²·sec andan average pore diameter of 7.5 μm. The nonwoven fabric was immersed ondistilled water bath at the room temperature, solidified andsufficiently washed to prepare a forward osmosis membrane supportconsisting of nonwoven fabric-reinforced polyacrylonitrile (PAN). Theforward osmosis membrane support thus obtained showed low poretortuosity due to the finger-like pore structure. The support was thenkept in ultrapure water for about one day to extract the solvent. Thesolvent-extracted support was immersed in an aqueous solution containing2 wt % of m-phenyldiamine (MPD) and 0.1 wt % ofN,N,N′,N′-tetramethyl-1,6-hexadiamine (TMHD) used as an aqueous additivefor 20 seconds, and removed of the water phase from the surface bycompression. The substrate was put in interfacial contact with anorganic solution containing 0.1 wt % of trimesoyl chloride (TMC) in anISOPAR solvent (Exxon Corp.) for 40 seconds to form a polyamide layerthrough an interfacial polymerization reaction between the compounds.The forward osmosis membrane thus obtained was dried under an airatmosphere for one minute, immersed in an aqueous solution containing0.2 wt % of an alkaline compound such as sodium carbonate at the roomtemperature for two hours and washed with distilled water to prepare aforward osmosis membrane having a composite membrane structure.

Examples 15 to 19

The procedures were performed to prepare a forward osmosis membrane of acomposite membrane structure in the same manner as described in Example14, excepting that the content of N,N,N′,N′-tetramethyl-1,6-hexadiamine(TMHD) used as an aqueous additive in Example 14 was varied.

Examples 20 to 23

The procedures were performed to prepare a forward osmosis membrane of acomposite structure in the same manner as described in Example 14,excepting that toluene sulfonic acid (TSA) was added toN,N,N′,N′-tetramethyl-1,6-hexadiamine (TMHD) used as an aqueous additivein Example 17 to vary the concentration of TSA.

Examples 24 and 25

The procedures were performed to prepare a forward osmosis membrane of acomposite structure in the same manner as described in Example 14,excepting that polyacrylonitrile-vinylacetate copolymer (P-CO-PAN) wasused as a material of the polymer layer instead of polyacrylonitrile(PAN) and that the content of P-CO-PAN was varied.

Examples 26 and 27

The procedures were performed to prepare a forward osmosis membrane of acomposite structure in the same manner as described in Example 24,excepting that there was used a nonwoven polyester fabric (nonwovenfabric 2) having porosity with an air permeability at least 10cc/cm²·sec and an average pore diameter at least 300 μm and that thecontent of P-CO-PAN was varied.

Comparative Example 3

The procedures were performed to prepare a forward osmosis membrane of acomposite structure in the same manner as described in Example 14,excepting that the polyamide layer in Example 14 was formed without anaqueous additive.

The compositions of the membranes prepared in Examples 14 to 27 arepresented in Table 4. The membranes were measured in regard to flux andback diffusion of salts in the same manner as described in Examples 1and 2. The measurement results are presented in Table 5.

TABLE 4 Preparation of Forward Osmosis membrane Membrane CompositionForward Osmosis Polyamide Layer Separation Support Aqueous Organic(Nonwoven Fabric/ Solution Solution Div. Polymer Layer) Monomer AqueousAdditive Monomer Example 14 Nonwoven Fabric 2 wt % 0.1 wt % TMHD 0.1 wt% Example 15 1/17.5 wt % PAN MPD 0.2 wt % TMHD TMC Example 16 0.5 wt %TMHD Example 17 1 wt % TMHD Example 18 1.2 wt % TMHD Example 19 2 wt %TMHD Example 20 1 wt % TMHD/0.5 wt % TSA Example 21 1 wt % TMHD/1 wt %TSA Example 22 1 wt % TMHD/1.1 wt % TSA Example 23 1 wt % TMHD/1.5 wt %TSA Comparative X Example 3 Example 24 Nonwoven Fabric 1 wt % TMHD/1 wt% 1/13 wt % P-CO- TSA PAN Example 25 Nonwoven Fabric 1/15 wt % P-CO- PANExample 26 Nonwoven Fabric 2/17 wt % P-CO- PAN Example 27 NonwovenFabric 2/15 wt % P-CO- PAN Comparative Nonwoven Fabric Example 1 3/18 wt% PSf Comparative CTA Example 2 Nonwoven fabric 1: air permeability of 2to 10 cc/cm² · sec Nonwoven fabric 2: air permeability at least 10cc/cm² · sec Nonwoven fabric 3: air permeability not more than 2 cc/cm²· sec PAN: Polyacrylonitrile P-CO-PAN: Polyacrylonitrile-vinylacetatecopolymer CO-PAN: Polyacrylonitrile-acrylic ester copolymer PSf:Polysulfone MPD: m-phenylenediamine TMHD:N,N,N′,N′-tetramethyl-1,6-hexadiamine TMC: Trimesoyl chloride TSA:Toluene sulfonic acid CTA: Cellulose Triacetate

TABLE 5 Property Assessment of Membrane Using 2M NaCl Draw SolutionChange of the Back Diffusion of Flux Conductivity Salts per Area Div.(GFD) ((μS/cm)/min) ((μS/cm)/min · cm²) Example 14 3.68 0.09 0.00375Example 15 5.78 0.1 0.00417 Example 16 6.07 0.16 0.00667 Example 17 5.523.74 0.156 Example 18 5.15 7.37 0.307 Example 19 X X X Example 20 6.40.11 0.00458 Example 21 6.14 0.1 0.00417 Example 22 6.48 0.08 0.00333Example 23 4.92 0.15 0.00625 Comparative 2.3 2.32 0.0967 Example 3Example 24 7.8 0.64 0.0267 Example 25 7.96 0.54 0.0225 Example 26 8.642.28 0.0950 Example 27 15.5 2.13 0.0888 Comparative 1.2 0.07 0.00292Example 1 Comparative 10 9.12 0.380 Example 2

As shown in Tables 4 and 5, the forward osmosis membrane (ComparativeExample 3) having a polyamide layer prepared without using an additiveto the aqueous solution was remarkably poor in flux relative to themembranes prepared in the examples of the present invention, but wassuperior in back diffusion variation of salts to the membrane(Comparative Example 2) consisting a single cellulose triacetate (CTA)material.

According to the property measurements, the membrane (ComparativeExample 1) having a polyamide layer formed on a polysulfone poroussupport as a support for a conventional reverse osmosis membrane showedan excellent effect of preventing back diffusion of salts but anextremely low flux, making the membrane impractical as a forward osmosismembrane driven by a concentration gradient difference.

Contrarily, the forward osmosis membranes prepared in the examples ofthe present invention were capable of preventing back diffusion of saltsof the draw solution and controllable in flux depending on the type ofnonwoven fabric and the type and content of the polymer constituting thesupport.

Hence, the forward osmosis membrane according to the embodiment of thepresent invention realized a forward osmosis membrane that had high fluxand low change of the conductivity per min not more than 9.0 (μS/cm)/minor not more than 0.375 (μS/cm)/min·cm², relative to the membraneconsisting of cellulose triacetate (salt diffusion assessment:conductivity 9.12 (μS/cm)/min), thereby minimizing diffusion of thesolutes of the draw solution and securing low back diffusion of salts.

As shown in Examples 24 and 27, it was possible to enhance the flux andto realize low back diffusion of salts when the membrane was preparedusing nonwoven fabric 2 (air permeability at least 10 cc/cm²·sec) ofhigh porosity with the same membrane composition excepting the use ofnonwoven fabric having a different porosity.

As described above, firstly, the present invention provides a forwardosmosis membrane for seawater desalination that has a composite membranestructure of sequentially laminated layers including a hydrophilicpolymer layer and a polyamide layer.

Secondly, the present invention provides a forward osmosis membrane forseawater desalination that has a composite membrane structure includinga hydrophilic polymer layer and a polyamide layer sequentially laminatedon a nonwoven fabric layer.

Thirdly, the present invention provides a method for preparing a forwardosmosis membrane for seawater desalination that facilitates an inflow ofwater from the feed water to the draw solution, realizes high waterpermeability in the direction of osmosis using a hydrophilic polymerlayer and exhibits contamination resistance using a polyamide layerformed on the hydrophilic polymer layer by interfacial polymerization.

While the present invention has been described with reference to theparticular illustrative embodiments, it is not to be restricted by theembodiments but only by the appended claims. It is to be appreciatedthat those skilled in the art can change or modify the embodimentswithout departing from the scope and spirit of the present invention.

We claim:
 1. A forward osmosis membrane for seawater desalination in aforward osmosis mode, said forward osmosis membrane having a compositemembrane structure of sequentially laminated layers comprising: ahydrophilic polymer layer having finger-like pores; and a polyamidelayer; wherein the forward osmosis membrane satisfies both (i) a backdiffusion of salts from a draw solution not more than 0.375(μS/cm)/min·cm² and (ii) a flux of 3 to 30 GFD as measured using a feedwater and a 2M NaCl draw solution in the forward osmosis mode; whereinthe hydrophilic polymer layer contains 10 to 25 wt % content of ahydrophilic polymer including any one selected from the group consistingof hydrophilic polyacrylonitrile, polyacrylonitrile-vinylacetatecopolymer (P-CO-PAN), and polyacrylonitrile-acrylic ester copolymer(CO-PAN); or the hydrophilic polymer containing 0.1 to 10 wt % of asulfonated polysulfone-based polymer represented by formula 1 incombination with a polysulfone-based polymer;

wherein A is any one functional group selected from:

B is any one functional group selected from:

m/(n+m) is 0.2 to 0.7; and x is 50 to 2,300.
 2. The forward osmosismembrane for seawater desalination according to claim 1, wherein thepolysulfone-based polymer is any one selected from the group consistingof polysulfone, polyethersulfone, and polyarylethersulfone, either aloneor as any mixture thereof.
 3. The forward osmosis membrane for seawaterdesalination according to claim 1, wherein the sulfonatedpolysulfone-based polymer is a compound represented by formula 2:

wherein m/(n+m) is 0.2 to 0.7; and x is 50 to 2,300.
 4. The forwardosmosis membrane for seawater desalination according to claim 1, whereinthe polymer layer has a thickness of 30 to 250 μm.
 5. The forwardosmosis membrane for seawater desalination according to claim 1, whereinan interfacial polymerization of an aqueous solution forms the polyamidelayer by further adding 0.01 to 2 wt % of the polyamine salt compound toan aqueous solution containing polyfunctional amine or alkylatedaliphatic amine and an organic solution containing a polyfunctional acylhalide compound.
 6. The forward osmosis membrane for seawaterdesalination according to claim 5, wherein the polyamine salt compoundis prepared from a tertiary polyamine and a strong acid at a molar ratioof 0.5˜2:1.
 7. The forward osmosis membrane for seawater desalinationaccording to claim 6, wherein the tertiary polyamine is any one selectedfrom the group consisting of 1,4-diazabicyclo[2,2,2]octane (DABCO),1,8-diazabicyclo[5,4,0]undec-7-ene (DBU),1,5-diazabicyclo[4,3,0]none-5-ene (DBN), 1,4-dimethylpiperazine,4-[2-(dimethylamino)ethyl]morpholine,N,N,N′,N′-tetramethylethylenediamine,N,N,N′,N′-tetramethyl-1,3-butanediamine,N,N,N′,N′-tetramethyl-1,4-butanediamine (TMBD),N,N,N′,N′-tetramethyl-1,3-propanediamine,N,N,N′,N′-tetramethyl-1,6-hexanediamine (TMHD),1,1,3,3-tetramethylguanidine (TMGU) andN,N,N′,N′,N″-pentamethyldiethylenetriamine.
 8. A forward osmosismembrane for seawater desalination in a forward osmosis mode, saidforward osmosis membrane having a composite membrane structure ofsequentially laminated layers comprising: a nonwoven fabric layer; ahydrophilic polymer layer having finger-like pores; and a polyamidelayer; wherein the forward osmosis membrane satisfies both (i) a backdiffusion of salts from a draw solution not more than 0.375(μS/cm)/min·cm² and (ii) a flux of 3 to 30 GFD as measured using a feedwater and a 2M NaCl draw solution in the forward osmosis mode; whereinthe hydrophilic polymer layer contains 10 to 25 wt % content of ahydrophilic polymer including any one selected from the group consistingof hydrophilic polyacrylonitrile, polyacrylonitrile-vinylacetatecopolymer (P-CO-PAN), and polyacrylonitrile-acrylic ester copolymer(CO-PAN); or the hydrophilic polymer containing 0.1 to 10 wt % of asulfonated polysulfone-based polymer represented by formula 1 incombination with a polysulfone-based polymer;

wherein A is any one functional group selected from:

B is any one functional group selected from:

m/(n+m) is 0.2 to 0.7; and x is 50 to 2,300.
 9. The forward osmosismembrane for seawater desalination according to claim 8, wherein thenonwoven fabric layer has an air permeability of at least 2cc/cm²·second.
 10. The forward osmosis membrane for seawaterdesalination according to claim 8, wherein the nonwoven fabric layer hasan average pore size of 1 to 600 μm.
 11. The forward osmosis membranefor seawater desalination according to claim 8, wherein the nonwovenfabric layer has a contact angle in the range of 0.1 to 74 degrees. 12.The forward osmosis membrane for seawater desalination according toclaim 8, wherein the nonwoven fabric layer has a thickness of 20 to 150μm.
 13. The forward osmosis membrane for seawater desalination accordingto claim 8, wherein the polysulfone-based polymer is any one selectedfrom the group consisting of polysulfone, polyethersulfone, andpolyarylethersulfone, either alone or as any mixture thereof.
 14. Theforward osmosis membrane for seawater desalination according to claim 8,wherein the sulfonated polysulfone-based polymer is a compoundrepresented by formula 2:

wherein m/(n+m) is 0.2 to 0.7; and x is 50 to 2,300.
 15. The forwardosmosis membrane for seawater desalination according to claim 8, whereinthe polymer layer has a thickness of 30 to 250 μm.
 16. The forwardosmosis membrane for seawater desalination according to claim 8, whereinan interfacial polymerization of an aqueous solution forms the polyamidelayer by further adding 0.01 to 2 wt % of the polyamine salt compound toan aqueous solution containing polyfunctional amine or alkylatedaliphatic amine and an organic solution containing a polyfunctional acylhalide compound.
 17. The forward osmosis membrane for seawaterdesalination according to claim 16, wherein the polyamine salt compoundis prepared from a tertiary polyamine and a strong acid at a molar ratioof 0.5˜2:1.
 18. The forward osmosis membrane for seawater desalinationaccording to claim 17, wherein the tertiary polyamine is any oneselected from the group consisting of 1,4-diazabicyclo[2,2,2]octane(DABCO), 1,8-diazabicyclo[5,4,0]undec-7-ene (DBU),1,5-diazabicyclo[4,3,0]none-5-ene (DBN), 1,4-dimethylpiperazine,4-[2-(dimethylamino)ethyl]morpholine,N,N,N′,N′-tetramethylethylenediamine,N,N,N′,N′-tetramethyl-1,3-butanediamine,N,N,N′,N′-tetramethyl-1,4-butanediamine (TMBD),N,N,N′,N′-tetramethyl-1,3-propanediamine,N,N,N′,N′-tetramethyl-1,6-hexanediamine (TMHD),1,1,3,3-tetramethylguanidine (TMGU) andN,N,N′,N′,N″-pentamethyldiethylenetriamine.